Field of the Invention
The present invention relates to a light emitting device, an optical write-in device, and an image forming device, and relates to a technology of controlling a light quantity of a light emitting element driven by a current using a thin film transistor with a high accuracy.
Description of the Related Art
An electrophotographic image forming device includes an optical write-in device in order to form an electrostatic latent image on a surface of a photoreceptor uniformly charged. In order to respond to a demand for reducing a size of an image forming device, a scanning type optical write-in device using a laser diode as a light emitting source is switching with a line optical type optical write-in device in which micro-dot light emitting elements are arranged in a line shape. In addition, in the line optical type optical write-in device using a semiconductor light emitting diode (LED) as a light emitting element, a LED array and a driving circuit for controlling light emitting elements are formed on different substrates from each other. Therefore, currently, cost of the line optical type optical write-in device cannot be reduced.
On the other hand, use of an organic LED (OLED) as a light emitting element allows the LED array and the driving circuit to be formed on the same substrate, and therefore cost of the optical write-in device can be reduced.
Such an optical write-in device (OLED print head (OLED-PH)) performs optical write-in by lighting many (for example, 15,000) light emitting elements arranged in a main scanning direction. Therefore, unevenness in a light quantity among the light emitting elements generates unevenness in a concentration in the main scanning direction not only in an electrostatic latent image but also in a toner image, and an excellent image cannot be achieved.
In a display device using an OLED, a light quantity variation is allowable to 30%. On the other hand, in an OLED-PH, a high accuracy for the light quantity variation is required, and even a light quantity variation of less than 1% has to be corrected. In order to remove the light quantity variation, preferably, a light sensor is not used in order to take advantage of merits of an OLED that cost of the optical write-in device can be reduced.
Examples of a technology of removing unevenness in a light quantity of a light emitting element include the following conventional technology. That is, as illustrated in FIGS. 14A and 14B, first, in a write-in period, a current digital to analogue converter (DAC) causes a driving current Id for emitting light at a desired light quantity to forcibly flow in a thin film transistor (TFT) 1402 for controlling a driving current amount supplied to a light emitting element 1401, and a capacitor 1403 stores a potential difference Vdg generated between a drain terminal and a gate terminal in the TFT 1402.
In a light emitting period, the voltage Vdg stored in the capacitor 1403 is applied between the gate terminal and the drain terminal in the TFT 1402 to cause the light emitting element 1401 to emit light at a desired light quantity (for example, refer to JP 2010-200514 A).
A threshold voltage Vth of the TFT 1402 has an initial variation. In addition, the drain current Id of the TFT 1402 is almost proportional to the threshold voltage Vth. Therefore, even when a constant voltage Vdg is applied to the TFT 1402, the driving current Id supplied is varied, and the light quantity of the light emitting element 1401 is not constant.
With respect to this problem, according to this conventional technology, the capacitor 1403 stores the voltage Vdg for supplying the driving current Id for causing the light emitting element 1401 to emit light at a desired light quantity, and the voltage Vdg is applied to the TFT 1402 in the light emitting period, and therefore a light quantity variation of the light emitting element 1401 can be suppressed.
In another conventional technology, all light emitting elements are caused to emit light under the same condition in advance, a light quantity of each of the light emitting elements is stored, and a driving condition is corrected for each of the light emitting elements according to the stored light quantity (refer to JP 2005-329634 A). Due to this, even when there is a variation in a light emitting efficiency α among the light emitting elements, the driving condition is corrected, for example, a driving current of a light emitting element having a large light quantity is reduced, and a driving current of a light emitting element having a small light quantity is increased when the light emitting elements are caused to emit light under the same condition. Therefore, a light quantity variation can be suppressed.
However, of the above two conventional technologies, the first conventional technology has the following problem caused by an initial variation in a forward direction voltage Vel of an OLED and a Vsd-Id characteristic between a source-drain voltage Vsd of a TFT and a drain current Id thereof.
The forward direction voltage Vel of the OLED has an initial variation. Therefore, as illustrated in FIGS. 14A and 14B, even when a voltage Vdd applied to a series circuit of the light emitting element 1401 and the TFT 1402 is constant, the source-drain voltage Vsd applied to the TFT 1402 satisfies Vsd=Vdd−Vel to generate a variation.
As the Vsd-Id characteristic of a TFT, when the source-drain voltage Vsd is increased in a saturation region, the drain current Id is also increased unlike a bipolar transistor or the like. Therefore, when the source-drain voltage Vsd is varied, the drain current Id is also varied, and therefore the first conventional technology cannot remove the light quantity variation of the light emitting element 1401.
Of the above two conventional technologies, the second conventional technology also has a problem caused by a forward direction voltage Vel of an OLED. That is, the forward direction voltage Vel of the OLED not only has an initial variation but also has a large temporal variation, and therefore has a variation of the driving current Id as described above. Therefore, even when only a temporal variation of the light emitting efficiency α is corrected, the light quantity variation cannot be removed.